The period of postpartum infertility in primiparous female primates is longer than the interval in multiparous females. First pregnancy and resultant lactation typically occur in female primates prior to the completion of the growth phase and therefore have implications for continued growth and subsequent reproductive capacity. Pregnancy and lactation produce specific changes in the neuroendocrine system controlling ovulation, and also increase the nutritional requirements of the female, both effects which may be exacerbated in young, sexually-mature females. We propose a series of longitudinal experiments using four groups of female rhesus monkeys to determine how age, parity and maternal nutritional status influence the duration and characteristics of postpartum infertility. Subjects will be studied in provisioned, outdoor housed social groups to assess how mother-infant interactions influence the lactational process. We will determine by measurement of body weight, percent body fat, and skeletal growth how age at first pregnancy and lactation influence the completion of the growth process. The studies will specify how first pregnancy and first lactation differentially influence the pattern of growth. A comparison of multiparous females to different-aged primaparous females during the lactational period will determine how age and parity affect patterns of 1) maternal care, 2) infant nursing behavior and physical growth, and 3) maternal endocrine status, measured by serum levels of prolactin, luteinizing hormone, follicle-stimulating hormone, and ovarian steroids. These analyses will elucidate how the duration of postpartum infertility in primiparous mothers is influenced by the nutritional costs of lactation, by infant nursing patterns, or by the interaction between these two variables. Finally, we will determine how parity and age influence the resumption of fertility and the capacity of females to conceive and maintain pregnancy. The characteristics of first lactation, including alterations in maternal nutritional status and basal insulin levels, will be related to the subsequent reproductive ability of the female. This project will further define how pregnancy and lactation influence the completion of growth and the parameters of subsequent reproductive activity in young primate mothers.